This invention relates to an apparatus and method for determining the relative values of the properties of optical fibers, and, more particularly, to the continuous measurement of the variations in the elastic properties of the optical fiber buffer layer.
Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds or thousands of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass fibers are generally fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches).
The buffer layer is a cured polymer. In the preferred practice, a thin uniform layer of an acrylate monomer that is curable or polymerizable in ultraviolet (UV) light is coated onto the glass strand of the optical fiber. The coated optical fiber is passed through a curing station having ultraviolet light sources such as mercury lamps that produce ultraviolet light at 350 nanometers wavelength. Polymerization or curing is accomplished in about 1 second of exposure.
In one application, the finished optical fiber is wound onto a cylindrical or slightly tapered conical bobbin with many turns adjacent to each other in a side by side fashion. After one layer is complete, another layer of optical fiber is laid on top of the first layer, and so on. The final assembly of the bobbin and the wound layers of optical fiber is termed a canister, and the mass of wound fiber is termed the fiber pack. When the optical fiber is later to be used, the optical fiber is paid out from the canister in a direction parallel to the axis of the cylinder.
It has been demonstrated that the elastic properties of the buffer layer can play an important role in the winding and payout characteristics of the optical fiber. The elastic properties depend upon the thickness and degree of curing of the buffer polymer material. Thus, the ultimate operability of the optical fiber in some applications is dependent upon the success in producing a very uniformly applied and cured buffer layer. Because of this interrelationship, it has been found necessary to inspect the optical fiber and its buffer layer over its entire length, which may be thousands of meters, to ascertain the degree of cure of the polymer of the buffer layer and its elastic properties.
Until now, there has been no reliable procedure for testing the state of curing of the buffer continuously along the length of the optical fiber. At the present time, there exist only static axial loading procedures wherein a segment of the optical fiber is statically loaded in tension to determine its elastic properties, from which the properties of the buffer layer can be inferred. This approach is not sufficient to meet future demands for the testing of large amounts of optical fiber.
There exists a need for determining the elastic properties of optical fibers along their entire lengths, and in particular for the elastic properties of the buffer layer as a way of determining its cure state. The present invention fulfills this need, and further provides related advantages.